AT commands are used to control MODEMs. AT is the abbreviation for Attention. These commands come from Hayes commands that were used by the Hayes smart modems. The Hayes commands started with AT to indicate the attention from the MODEM. The dial up and wireless MODEMs (devices that involve machine to machine communication) need AT commands to interact with a computer. These include the Hayes command set as a subset, along with other extended AT commands.
AT commands with a GSM/GPRS MODEM or mobile phone can be used to access following information and services:
1. Information and configuration pertaining to mobile device or MODEM and SIM card.
2. SMS services.
3. MMS services.
4. Fax services.
5. Data and Voice link over mobile network.
The Hayes subset commands are called the basic commands and the commands specific to a GSM network are called extended AT commands.
Types of AT Commands:
There are four types of AT commands:
1) Test commands - used to check whether a command is supported or not by the MODEM.
SYNTAX: AT<command name>=?
For example: ATD=?
2) Read command - used to get mobile phone or MODEM settings for an operation.
SYNTAX: AT<command name>?
For example: AT+CBC?
3) Set commands - used to modify mobile phone or MODEM settings for an operation.
SYNTAX: AT<command name>=value1, value2, …, valueN
Some values in set commands can be optional.
For example: AT+CSCA=”+9876543210”, 120
4) Execution commands - used to carry out an operation.
SYNTAX: AT<command name>=parameter1, parameter2, …, parameterN
The read commands are not available to get value of last parameter assigned in execution commands because parameters of execution commands are not stored.
For example: AT+CMSS=1,”+ 9876543210”, 120